Plane light source and LCD backlight unit having the same

ABSTRACT

There are provided a plane light source and an LCD backlight unit having the same. A plane light source including light emitting device matrixes each having a plurality of light emitting devices arranged in rows and columns on a substrate according to an aspect of the invention includes: a first matrix having a plurality of light emitting devices arranged in rows and columns; and a second matrix having a plurality of light emitting devices arranged in rows and columns, the light emitting devices each located within a rectangle formed by four adjacent light emitting devices included in the first matrix, and forming angles θ satisfying the condition of 45°≦θ≦55° therebetween on the basis of a horizontal direction, wherein among pitches between one light emitting devices included in the light emitting device matrixes and another lighting light emitting device adjacent to the light emitting device, a pitch P1 between the light emitting device and the light emitting device diagonally across from the light emitting device satisfies the condition of 25 mm≦P 1 ≦29 mm, and a pitch P 2  between the light emitting device and another light emitting device located in a horizontal direction satisfies the condition of 34 mm≦P 2 ≦38 mm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.12/199,327, filed Aug. 27, 2008, which claims priority from KoreanPatent Application No. 2007-0087401, filed on Aug. 30, 2007, in theKorean Intellectual Property Office, the disclosures of which areincorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to plane light sources and LCD backlightunits having the same, and more particularly, to a plane light sourcethat has high efficiency and can reduce the number of light emittingdevices by optimizing the arrangement and pitch of the light emittingdevices, and an LCD backlight unit having the same.

2. Description of the Related Art

In general, when a current is applied to a semiconductor light emittingdiode (LED), the semiconductor LED can generate light of various colorsby recombination of electrons and holes in a p-n junction between p-typeand n-type semiconductors. When compared with a filament-based lightemitting device, the LED has a longer lifespan, lower power consumption,excellent initial driving characteristics, higher vibration resistance,and higher tolerance for repetitive power switching. Thus, there hasbeen an increasing demand for LEDs. Nowadays, group III nitridesemiconductors that can emit light in a short-wavelength region,including blue, have attracted attention.

In the related art, since a cold cathode fluorescent lamp (CCFL), whichis used as a light source for an LCD backlight unit, uses mercury gas,environmental contamination may be caused. Furthermore, the CCFL has lowresponse speed and low color reproducibility, and may not allow areduction in size, thickness, and weight of an LCD panel.

Compared to the CCFL, a light emitting diode (LED) isenvironment-friendly, has a response speed of several nanoseconds toachieve high-speed response and be effective for a video signal stream,and allows impulsive driving. Further, the LED has a colorreproducibility of 100% or more, varies in luminance, color temperature,and the like by controlling the intensity of light of the red, green,and blue LEDs, and can result in a reduction in size, thickness, andweight of the LCD panel. Accordingly, the LED has been widely used as alight source for the backlight unit of the LCD panel or the like.

An LCD backlight using an LED may be divided into an edge type backlightand a direct type backlight according to the position of a light source.In a case of the edge type backlight, a bar-shaped light source having awidth larger than its length is positioned at the side thereof and emitslight onto a front surface of the LCD panel by using a light guidepanel. In a case of the direct type backlight, a plane light source ispositioned at a lower part of the LCD panel, and light is directlyirradiated onto a front surface of the LCD panel from the plane lightsource that has almost the same area as the LCD panel.

FIG. 1 is a view illustrating the arrangement of light emitting devicesof a plane light source according to the related art.

As shown in FIG. 1, according to the related art, a plane light source100 that is used in a direct type LCD panel includes a plurality of LEDs102 that are arranged in rows and columns on a substrate 101. In thiscase, each four adjacent LEDs 102 of the plurality of LEDs 102 form arectangle.

In the plane light source 100, the substrate 101 is divided into eightblocks. Eighteen LEDs 102 are arranged in each block, and thus, onehundred forty four LEDs 102 are included in the entire substrate 101.Here, as shown in FIG. 1, the entire substrate 101 is 453×124 mm.

However, the above arrangement requires a larger number of LEDs that areused to cover the same light emitting area than necessary.

Further, an area adjacent to the LEDs 102 has much higher brightnessthan an area distant from the LEDs 102, that is, the center of therectangle formed by the four LEDs 102. That is, when a large number ofLEDs 102 are arranged, a uniformity of brightness can be achieved.However, when the number of LEDs is reduced to improve the efficiency asdescribed above, the pitch between the neighboring LEDs becomes larger.Thus, variation may occur in brightness distribution.

Therefore, in terms of a plane light source used in the LCD panel or thelike, there is a need for a method of improving the efficiency of theplane light source by reducing the number of light emitting devices usedin the plane light source, and achieving the uniformity of luminance,that is, little difference in brightness.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a plane light source thathas high efficiency and can reduce the number of a plurality of lightemitting devices by optimizing the arrangement and pitch of the lightemitting devices, and an LCD backlight unit having the same.

According to an aspect of the present invention, there is provided aplane light source including light emitting device matrixes each havinga plurality of light emitting devices arranged in rows and columns on asubstrate, the plane light source comprising: a first matrix having aplurality of light emitting devices arranged in rows and columns; and asecond matrix having a plurality of light emitting devices arranged inrows and columns, the light emitting devices each located within arectangle formed by four adjacent light emitting devices included in thefirst matrix, and forming angles θ satisfying the condition of 45°≦θ≦55°therebetween on the basis of a horizontal direction, wherein amongpitches between one light emitting devices included in the lightemitting device matrixes and another lighting light emitting deviceadjacent to the light emitting device, a pitch P1 between the lightemitting device and the light emitting device diagonally across from thelight emitting device satisfies the condition of 25≦mm≦P1≦29 mm, and apitch P2 between the light emitting device and another light emittingdevice located in a horizontal direction satisfies the condition of 34mm≦P2≦38 mm.

Each of the light emitting devices included in the second matrix may belocated at the center of the rectangle.

The light emitting device may emit white light.

The plane light source may further include a diffusion sheet disposedalong a light emission path of the light emitting devices.

The light emitting device may be a light emitting diode (LED).

According to another aspect of the present invention, there is providedan LCD backlight unit attached to a rear surface of an LCD panel, theLCD backlight unit including: the plane light source; a diffusion sheetlocated above the plane light source and uniformly diffusing lightincident from the plane light source; and at least one light collectingsheet located above the diffusion sheet and collecting light diffused bythe diffusion sheet in a direction perpendicular to the plane of the LCDpanel.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view illustrating the arrangement of lightemitting devices of a plane light source according to the related art;

FIG. 2 is a schematic view illustrating the arrangement of lightemitting devices of a plane light source according to an exemplaryembodiment of the invention;

FIG. 3 is an enlargement view illustrating one block in the plane lightsource, shown in FIG. 2;

FIG. 4 is an exploded side view illustrating an LCD backlight unitaccording to another exemplary embodiment of the invention; and

FIG. 5 is a view illustrating luminance distribution of the plane lightsource according to the embodiment of the invention (shown in FIG. 2)and the plane light source according to the related art (shown in FIG.1).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary embodiments of the present invention will now be described indetail with reference to the accompanying drawings.

The invention may however be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the invention to thoseskilled in the art. In the drawings, the shapes and dimensions may beexaggerated for clarity, and the same reference numerals will be usedthroughout to designate the same or like components.

FIG. 2 is a schematic view illustrating the arrangement of lightemitting devices of a plane light source according to an exemplaryembodiment of the invention.

A plane light source 200 according to this embodiment includes aplurality of light emitting devices 202 that are arranged on a substrate201.

As shown in FIG. 2, the light emitting devices 202 are arranged in amatrix having rows and columns in a zigzag manner. A first matrixincludes a plurality of light emitting devices that are arranged in rowsand columns in a straight line. A second matrix having the sameconfiguration as the first matrix is located within the first matrix.That is, each of the light emitting devices of the second matrix islocated within a rectangle formed by each four light emitting devicesincluded in the first matrix.

Like the general plane light source, shown in FIG. 1, the plane lightsource 200 according to this embodiment includes eight blocks. Asubstrate 201 is 153×124 mm.

As shown in FIG. 2, each block has nine light emitting devices 202, andthus, seventy two light emitting devices 202 are arranged on thesubstrate 201. Compared to the plane light source, shown in FIG. 1,which includes the one hundred forty four light emitting devices, theplane light source according to this embodiment can reduce the number oflight emitting devices by approximately 50%. Therefore, manufacturingcosts can be remarkably reduced.

However, in this embodiment, even though the number of light emittingdevices is reduced, a reduction in luminance can be prevented since thearrangement of the light emitting devices 202 is optimized. This will bedescribed below with reference to FIG. 5.

The arrangement of light emitting devices according to this embodimentwill be described in detail with reference to FIG. 3.

FIG. 3 is an enlarged view illustrating one block of the plane lightsource according to the related art, shown in FIG. 2.

In this embodiment, each of the light emitting devices included in thesecond matrix meets the four light emitting devices of the first matrixat angles of θ, where the angles θ satisfy the condition of 45°≦θ≦55°.As a result, in the zigzag arrangement, a position of each of the lightemitting devices arranged within a rectangle formed by the four lightemitting devices is balanced with respect to positions of the four lightemitting devices. Therefore, compared to the checkered arrangement ofthe light emitting devices in the related art, the uniformity ofluminance distribution can be improved in this embodiment, which will bedescribed in detail below.

In this embodiment, in addition to the angles between the light emittingdevices, a pitch P1 between the adjacent light emitting devices 202 anda pitch P2 between the adjacent light emitting devices 202 are optimizedto ensure the uniformity of luminance of the plane light source 200.

That is, in this embodiment, among the neighboring light emittingdevices 202, the pitch P1 between the light emitting device 202 and thelight emitting device 202 diagonally across from it satisfies thecondition of 25 mm≦P1≦29 mm. At the same time, among the neighboringlight emitting devices 202, the pitch. P2 between the light emittingdevices 202 and the light emitting device 202 located in a horizontaldirection satisfies the condition of 34 mm≦P2≦38 mm.

When each of the pitches P1 and P2 between the light emitting devices202 is smaller than the above pitch conditions, the luminance and theuniformity of luminance distribution can be improved, but at the sametime, the number of light emitting devices used in the substrate havingthe same area also increases. As a result, it becomes difficult toachieve the object of the invention to reduce the number of lightemitting devices and prevent the reduction of the luminance and theuniformity of luminance distribution.

On the other hand, when each of the pitches P1 and P2 between the lightemitting devices is greater than the above pitch conditions, theopposite result will be obtained. That is, when the pitch P1 and thepitch P2 are greater than 29 mm and 38 mm, respectively, the number oflight emitting devices can be reduced, but at the same time, theluminance and the uniformity of luminance distribution may besignificantly reduced.

In this embodiment, the pitches P1 and P2 between the light emittingdevices are the most important factor to be considered. Compared to therelated art, when the number of light emitting devices used in the planelight source is reduced, one light emitting device is inevitably spacedapart from another light emitting device. For example, as shown in FIG.1, in a case of the arrangement of the light emitting devices generallyused, the pitch between the neighboring light emitting devices is in therange of 23.00 to 27.00 mm in width and in the range of 20.00 to 27.00mm in height. This pitch is smaller than the pitch between the lightemitting devices according to this embodiment.

As such, in the plane light source according to this embodiment, thegreater pitch between the light emitting devices is, the lower luminanceis. Furthermore, the uniformity of luminance distribution is alsoreduced since a dark portion having very low luminance may occur aroundthe middle between the light emitting devices.

In this embodiment, the proposed numerical ranges of the pitches P1 andP2 are determined so that the number of light emitting devices can bereduced to the lowest possible number, and the reduction of luminancecaused by the reduction in number of light emitting devices can beprevented.

In this embodiment, on the assumption that the angle θ is 45° on thebasis of any one light emitting device L, and other light emittingdevices in the horizontal direction also satisfy the condition of theangle θ, the pitches P1 and P2 can easily be determined by usingtrigonometric functions. For example, when the pitch P1 is 25 mm, alength approximately half of the pitch P2 corresponds to a length ofeach of the equal sides that form an isosceles triangle with thehypotenuse corresponding to the pitch P1. In this embodiment, the pitchP2 is approximately 35 mm.

As described above, the light emitting devices having rows and columnsare not arranged in a straight line but in a zigzag manner. Therefore,the number of light emitting devices can be reduced by 15 to 50% withrespect to the same light emitting area.

In general, the light emitting device 202 may be formed of an LED.However, the invention is not limited thereto. Preferably, the lightemitting device 202 may be formed of a device capable of emitting whitelight to be widely used as a light source.

Specifically, in the light emitting device 202, blue light is emittedfrom an active layer that constitutes the LED, and a yellow phosphormaterial may be applied along a light emission path of the LED.

The plane light source can be used in an LCD backlight unit 300 thatilluminates an LCD panel from the back.

FIG. 4 is an exploded side view illustrating the LCD backlight unit 300according to an exemplary embodiment of the invention. As shown in FIG.4, the LCD backlight unit 300 according to this embodiment that isattached to a rear surface of the LCD panel includes the above-describedplane light source 1 and a diffusion sheet 316 that is disposed abovethe plane light source 1 and uniformly diffuses light incident from theplane light source 1.

The LCD backlight unit 300 may also include at least one lightcollecting sheet 314. The light collecting sheet 314 is disposed abovethe diffusion sheet 316 and collects light, diffused by the diffusionsheet 316, in a direction perpendicular to the plane of the LCD panel310. Furthermore, the LCD backlight unit 300 may include a protectivesheet 312 that is disposed above the light collecting sheet 314 andprotects an optical structure located below the LCD panel 310.

The plane light source 1 includes a substrate 351 and a plurality oflight emitting devices 352 that are arranged in a matrix format on thesubstrate 351 as described in the embodiment of the invention. The planelight source 1 includes a side wall 354 and a reflective layer 356. Theside wall 354 is located at the edges of an upper surface of thesubstrate 351, encompasses the light emitting devices 352, and has aninclined surface in a direction in which the light emitting devices 352are arranged. The reflective layer 356 is formed on the upper surface ofthe substrate 351, and reflects light, emitted from the light emittingdevices 252, upward.

Preferably, a reflective material 354 a may also be applied to theinclined surface of the side wall 354 so that laterally emitted lightcan be emitted upward.

Meanwhile, the diffusion sheet 316 that is located above the plane lightsource 1 diffuses light incident from the plane light source 1 toprevent local concentration of light. Further, the diffusion sheet 316controls the direction of the light moving towards the first lightcollecting sheet 314 a to reduce an angle of inclination with respect tothe first light collecting sheet 314 a. In this case, as describedabove, the pitch between the diffusion sheet 316 and the light emittingdevices 352 included in the plane light source 1 corresponds to anoptical length l of the Equation 1. The pitch may be determinedaccording to the arrangement of the light emitting devices 352. On thecontrary, the arrangement of the light emitting devices 352 may bedetermined by the pitch between the light emitting devices 352 and thediffusion sheet 316.

Each of the first light collecting sheet 314 a and the second lightcollecting sheet 314 b has a predetermined arrangement of triangularprisms on an upper surface thereof. The prisms of the first lightcollecting sheet 314 a and the prisms of the second light collectingsheet 314 b cross each other at a predetermined angle (for example,90°). Each of the first and second light collecting sheets 314 a and 314b collects light diffused by the diffusion sheet 316 in a directionperpendicular to the plane of the LCD panel 310. This allows almostperfect perpendicular incidence of light, passing through the first andsecond light collecting sheets 314 a and 314 b, with respect to theprotective sheet 312. Therefore, the light passing through the first andsecond light collecting sheets 314 a and 314 b mostly moves in aperpendicular direction to thereby obtain the uniform luminancedistribution of the protective sheet 312. In FIG. 4, two lightcollecting sheets are used. However, only one light collecting sheet canbe used if necessary.

The protective sheet 312 located above the second light collecting sheet314 b protects the surface of the second light collecting sheet 314 band at the same time, diffuses light to obtain the uniform distributionof light. The LCD panel 310 is installed above the protective sheet 312.

In this embodiment, the LCD backlight unit 300 uses the plane lightsource 1 that obtains the uniform luminance distribution of the emittedlight to thereby reduce a variation in brightness that changes accordingto different regions of the LCD panel.

Finally, FIG. 5 is a view illustrating luminance distribution measuredin the plane light source according to the embodiment of the invention(the embodiment of FIG. 2) and the plane light source according to therelated art (the embodiment of FIG. 1). In FIG. 5, in order to make acomparison in luminance and uniformity of luminance distribution betweenthe plane light sources, the plane light sources are provided inbacklight units, shown in FIG. 4, and luminous characteristics thereofare shown.

In a graph shown on the upper side of FIG. 5, the luminance distributionof light emitted from the plane light source according to the relatedart is shown in a direction perpendicular to a light emitting direction.In a graph shown on the lower side of FIG. 5, the luminance distributionof light emitted from the plane light source according to the embodimentof the invention is shown. Here, it may be considered that a horizontalaxis of each graph, shown in FIG. 5, indicates a horizontal direction ofthe light emitting device matrixes of the plane light source.

Referring to the two graphs, shown in FIG. 5, the plane light source isshown to have a luminance of approximately 12000 Cd/m² according to therelated art, and the plane light source is shown to have a luminance ofapproximately 10000 Cd/m² according to the embodiment of the invention.Therefore, in comparison with the related art, the luminance of theplane light source is reduced by approximately 17%. In consideration ofthe above description, the number of light emitting devices arranged onthe substrate is reduced, and thus it can be said that the efficiency issignificantly improved compared to the related art.

Further, even though the average pitch between the light emittingdevices is increased due to the reduced number of light emittingdevices, as compared to the related art, the uniformity of the luminancedistribution is not brought into question at all. Rather, the uniformityof the luminance distribution is improved.

As set forth above, according to the exemplary embodiments of theinvention, a plane light source that has high efficiency and reduces thenumber of light emitting devices by optimizing the arrangement and pitchof the light emitting devices can be provided, and an LCD backlight unithaving the same can also be provided. Further, according to theexemplary embodiment of the invention, the uniform luminance of theentire plane light source can be achieved.

While the present invention has been shown and described in connectionwith the exemplary embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

What is claimed is:
 1. A plane light source including light emittingdevice matrixes each having a plurality of light emitting devicesarranged in rows and columns on a substrate, the plane light sourcecomprising: a first matrix having a plurality of light emitting devicesarranged in rows and columns; a second matrix having a plurality oflight emitting devices arranged in rows and columns, the light emittingdevices each located within a rectangle formed by four adjacent lightemitting devices included in the first matrix, and forming angles θsatisfying the condition of 45°≦θ≦55° therebetween on the basis of ahorizontal direction; a side wall located at the edges of an uppersurface of the substrate encompassing the plurality of light emittingdevices, the side wall having an inclined surface; and a reflectivelayer formed on the substrate, the reflective layer reflecting from theplurality of light emitting devices, wherein among pitches between onelight emitting devices included in the light emitting device matrixesand another lighting light emitting device adjacent to the lightemitting device, a pitch P1 between the light emitting device and thelight emitting device diagonally across from the light emitting devicesatisfies the condition of 25 mm≦P1≦29 mm, and a pitch P2 between thelight emitting device and another light emitting device located in ahorizontal direction satisfies the condition of 34 mm≦P2≦38 mm.
 2. Theplane light source of claim 1, wherein each of the light emittingdevices included in the second matrix is located at the center of therectangle.
 3. The plane light source of claim 1, wherein the lightemitting device emits white light.
 4. The plane light source of claim 1,further comprising a diffusion sheet disposed along a light emissionpath of the light emitting devices.
 5. The plane light source of claim1, wherein the light emitting device is a light emitting diode (LED).